STUDIES OF THE SURFACE-COMPOSITION OF PHENYL AND CYANOPROPYL BONDED PHASES UNDER REVERSED-PHASE LIQUID-CHROMATOGRAPHIC CONDITIONS USING ALKANOATE AND PERFLUOROALKANOATE ESTERS

Two polar bonded phases, phenyl-silica and cyanopropyl-silica, have been studied under reversed-phase conditions using homologues of alkanoate and perfluoroalkanoate esters as test solutes. For comparative purposes a similarly prepared n-butyl phase also has been studied under equivalent conditions. Retention measurements have been carried out as a function of mobile-phase composition for water-methanol and water-acetonitrile and incremental methylene (-CH2-) and perfluoromethylene (-CF2-) selectivities calculated. These latter data have been used to estimate the thermodynamic constant K(wo), which describes the sorption equilibrium of solvents between mobile and stationary phases assuming ideality of these phases. For water-methanol, differences in composition between the mobile and surface phases due to preferential intercalation are small. However, the data indicate a significant sorption excess of acetonitrile by the bonded phases. This preferential sorption of acetonitrile is greatest for alkyl phases and decreases with increasing polarity of the bonded phase (i.e., cyano-silica < phenyl-silica < alkyl-silica).